Add Rails 4 Appraisal

Rails 4 includes strong parameters and removed att_accessible (protected
attributes). I removed the strong_parameters development dependency and
added it to all non-4.0 appraisals. I also added the
'protect_attributes' gem to the 4.0 appraisal because the test suite
examples make heavy use of attr_accessible.

All 3.x tests still pass. Many failures in 4.0 to attack
Fix render_with_layout_matcher on Rails 4.

Rails 4 moved the layouts ivar from `@layouts` to `@_layouts`.

This change introduces what I have (for now) called the 'RailsShim'. The
idea is that this single class will house the rail version workarounds,
which can then be reused through the code. This paid off here because
both the spec and the implementation need access to the layouts ivar.
Fix set_the_flash_matcher failures related to ivar

The flashes ivar has changed in Rails 4. I have added a shim to get the
proper ivar name. This fixed many failures in the spec for this matcher.
I believe the lone remaining failure is unrelated to the ivar name.
Use `get` instead of `match` in routes

match is not supported in Rails 4. This still leaves several failures in
the route_matcher, but these seem to be related to the underlying route
assertion relying on minitest methods. Need to do more research here...

https://github.com/rspec/rspec-rails/issues/635
https://github.com/rspec/rspec-rails/issues/679